1. Field of the Invention
This invention relates to a brake device for a shutter, and, more particularly to a brake device for stopping run of shutter blades of a focal plane shutter for cameras.
2. Description of the Related Art
An example of a conventional brake device of a shutter will now be described with reference to FIGS. 5 and 6. In this device, a blade driving lever 108 connected to the shutter blades to transmit a driving force to the shutter blades is rotated clockwise, viewed in the figures, around a shaft P so that the shutter blades are made run. Immediately before completion of running of the shutter blades the blade driving lever 108 which can be, as described above, rotated is brought into contact with a brake lever 113 so that the rotation of the blade driving lever 108 is rapidly stopped by the frictional force which is given the brake lever 113. The position at which the blades are stopped is finally determined by stoppage of rotation of the brake lever 113 by abutting a bent and raised portion 113c thereof against a stopper rubber 118. The structure which can cause the brake lever 113 to be rotated under a predetermined frictional state is exemplified in this example by that: the brake lever 113 is sandwiched and held between an upper and a lower brake sheets 119 utilizing spring force from a leaf spring 122 via a fixed plate 120; and thus the rotation under a predetermined resistance can be realized by frictional resistance obtained by the above-described sandwiching and holding. A collar 121 transmits the urging force from the leaf spring 122 as the sandwiching pressure which acts between the frictional surface of the brake lever 113 and that of the brake sheets 119. Furthermore, the outside surface of the collar 121 is arranged to act as a guide rod for a spring 116.
A buffer member 224 formed by an elastic material is interposed between a blade driving lever 208 and a brake lever 213 in another conventional device shown in FIGS. 7 and 8 so that the shock due to abutment between the blade driving lever 208 and the brake lever 213 is reduced, the abutment occurring immediately before completion of the running of the blades.
In the description of the conventional device, some components which are substantially the same as those of the device according to an embodiment of the present invention to be described later will be illustrated by giving reference numerals to which 100 (in a case of FIGS. 5 and 6) or 200 (in a case of FIGS. 7 and 8) are added as an alternative to the detailed description of them.
However, in the sample shown in FIGS. 5 and 6, since both the blade driving lever 108 and the brake lever 113 are usually made of a metal, a large impact between two metallic parts is generated when the blade driving lever 108 which is driven at high speed is brought into abutment with the brake lever 113. As a result of such shock at the initial stage of stopping process, problems arise in that the durability of the blades can be deteriorated and/or uncomfortable high tone noise due to the metal impact will be generated.
Furthermore, in the example shown in FIGS. 7 and 8, although an advantage can be obtained in that the shock occurring at the initial stage of the stopping process can be absorbed and the high tone noise can be prevented thanks to the presence of the buffer member 224, a problem, on the contrary, arises in that the position at which the running of the blades is completed becomes unstable. Namely, the blade driving lever 208 is first brought into abutment with the brake lever 213 via the yielding buffer member 224. Next, the brake lever 213 is brought into abutment with the stopper rubber 218 with maintaining the state of abutment realized with the yielding buffer member 224 so that the final stop position of the blade driving lever 208 can be determined. As a result of the above-described structure, the deformations of two elastic members (the buffer member 224 and the stopper rubber 218) are added to each other, the two members being disposed at the intermediate positions of the passage through which force is transmitted. This will cause instability in determining the above-described position.
Such positional instability in completion of running of the shutter will sometimes cause a very serious problem in a shutter for cameras. For example, if the amount of deformation of the buffer member 224 is smaller than the amount estimated, the portion of the leading blade group in which slit is formed is remained in the aperture, causing a so-called "jump". In the trailing blade group, the portion in which slits are formed cannot drop but remains within the aperture, as a result of this, a problem arises in that a gap is formed. This leads to a non-uniform exposure, if the worst happens, it causes leak of light. On the other hand, if the amount of deformation of the buffer member 224 is larger than the amount estimated, the blade group overruns, causing for the structure of a parallel link mechanism which generally supports the blade group in such a manner that the blade group can run to be applied with excess force. It will cause deterioration in durability, and if in an extreme case, the structure can be destroyed.
Since the above-described buffer member 224 serves to absorb the action of impacting force, load at the time of action is relatively large with respect to the stopper rubber 218 which is engaged with the brake lever 213 for finally stopping the same after the brake lever 213 has been sufficiently decelerated. As a result of this, change in amount of deformation in accordance with time elapse is larger than that of the stopper rubber 218. Therefore, it is very difficult to control the proper amount of deformation through the overall time period for use, estimate the proper amount of deformation at the time of designing the device, and secure the precision of the components of the buffer member and accuracy of the components at assembly.